xfs: skip delalloc COW blocks in xfs_reflink_end_cow

The iomap direct I/O code issues a single ->end_io call for the whole
I/O request, and if some of the extents cowered needed a COW operation
it will call xfs_reflink_end_cow over the whole range.

When we do AIO writes we drop the iolock after doing the initial setup,
but before the I/O completion.  Between dropping the lock and completing
the I/O we can have a racing buffered write create new delalloc COW fork
extents in the region covered by the outstanding direct I/O write, and
thus see delalloc COW fork extents in xfs_reflink_end_cow.  As
concurrent writes are fundamentally racy and no guarantees are given we
can simply skip those.

This can be easily reproduced with xfstests generic/208 in always_cow
mode.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/xfs_reflink.c

@@ -693,14 +693,12 @@ xfs_reflink_end_cow(
 		if (!del.br_blockcount)
 			goto prev_extent;
 
-		ASSERT(!isnullstartblock(got.br_startblock));
-
 		/*
-		 * Don't remap unwritten extents; these are
-		 * speculatively preallocated CoW extents that have been
-		 * allocated but have not yet been involved in a write.
+		 * Only remap real extent that contain data.  With AIO
+		 * speculatively preallocations can leak into the range we
+		 * are called upon, and we need to skip them.
 		 */
-		if (got.br_state == XFS_EXT_UNWRITTEN)
+		if (!xfs_bmap_is_real_extent(&got))
 			goto prev_extent;
 
 		/* Unmap the old blocks in the data fork. */